# std::ranges::mismatch, std::ranges::mismatch_result

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::input_iterator I1, std::sentinel_for<I1> S1, std::input_iterator I2, std::sentinel_for<I2> S2, class Pred = ranges::equal_to, class Proj1 = std::identity, class Proj2 = std::identity > requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2> constexpr mismatch_result<I1, I2> mismatch( I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} ); | (1) | (since C++20)
template< ranges::input_range R1, ranges::input_range R2, class Pred = ranges::equal_to, class Proj1 = std::identity, class Proj2 = std::identity > requires std::indirectly_comparable< ranges::iterator_t<R1>, ranges::iterator_t<R2>, Pred, Proj1, Proj2> constexpr mismatch_result<ranges::borrowed_iterator_t<R1>, ranges::borrowed_iterator_t<R2>> mismatch( R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} ); | (2) | (since C++20)
Helper types |  |
template< class I1, class I2 > using mismatch_result = ranges::in_in_result<I1, I2>; | (3) | (since C++20)

Returns the first mismatching pair of projected elements from two ranges: one defined by `[`first1`,` last1`)` or r1 and another defined by `[`first2`,` last2`)` or r2.

1) Elements are compared using the given binary predicate p.
2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first1, last1 | - | an iterator-sentinel pair denoting the first range of the elements to compare
r1 | - | the first range of the elements to compare
first2, last2 | - | an iterator-sentinel pair denoting the second range of the elements to compare
r2 | - | the second range of the elements to compare
pred | - | predicate to apply to the projected elements
proj1 | - | projection to apply to the first range of elements
proj2 | - | projection to apply to the second range of elements

### Return value

`ranges::mismatch_result` with iterators to the first two non-equal elements.

If no mismatches are found when the comparison reaches last1 or last2, whichever happens first, the object holds the end iterator and the corresponding iterator from the other range.

### Complexity

At most std::min(last1 - first1, last2 - first2) applications of the predicate and corresponding projections.

### Possible implementation

    struct mismatch_fn
    {
        template<std::input_iterator I1, std::sentinel_for<I1> S1,
                 std::input_iterator I2, std::sentinel_for<I2> S2,
                 class Pred = ranges::equal_to,
                 class Proj1 = std::identity, class Proj2 = std::identity>
        requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
        constexpr std::mismatch_result<I1, I2>
            operator()(I1 first1, S1 last1, I2 first2, S2 last2,
                       Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const
        {
            for (; first1 != last1 && first2 != last2; ++first1, (void)++first2)
                if (not std::invoke(pred, std::invoke(proj1, *first1),
                                          std::invoke(proj2, *first2)))
                    break;

            return {first1, first2};
        }

        template<ranges::input_range R1, ranges::input_range R2,
                 class Pred = ranges::equal_to,
                 class Proj1 = std::identity, class Proj2 = std::identity>
        requires std::indirectly_comparable<ranges::iterator_t<R1>, ranges::iterator_t<R2>,
                                            Pred, Proj1, Proj2>
        constexpr ranges::mismatch_result<ranges::borrowed_iterator_t<R1>,
                                          ranges::borrowed_iterator_t<R2>>
            operator()(R1&& r1, R2&& r2, Pred pred = {},
                       Proj1 proj1 = {}, Proj2 proj2 = {}) const
        {
            return (*this)(ranges::begin(r1), ranges::end(r1),
                           ranges::begin(r2), ranges::end(r2),
                           std::ref(pred), std::ref(proj1), std::ref(proj2));
        }
    };

    inline constexpr mismatch_fn mismatch;

### Example

This program determines the longest substring that is simultaneously found at the very beginning and at the very end of the given string, in reverse order (possibly overlapping).

Run this code

    #include <algorithm>
    #include <iostream>
    #include <ranges>
    #include <string_view>

    [[nodiscard]]
    constexpr std::string_view mirror_ends(const std::string_view in)
    {
        const auto end = std::ranges::mismatch(in, in | std::views::reverse).in1;
        return {in.cbegin(), end};
    }

    int main()
    {
        std::cout << mirror_ends("abXYZba") << '\n'
                  << mirror_ends("abca") << '\n'
                  << mirror_ends("ABBA") << '\n'
                  << mirror_ends("level") << '\n';

        using namespace std::literals::string_view_literals;

        static_assert("123"sv == mirror_ends("123!@#321"));
        static_assert("radar"sv == mirror_ends("radar"));
    }

Output:

    ab
    a
    ABBA
    level

### See also

ranges::equal (C++20) | determines if two sets of elements are the same (algorithm function object)
ranges::findranges::find_ifranges::find_if_not (C++20)(C++20)(C++20) | finds the first element satisfying specific criteria (algorithm function object)
ranges::lexicographical_compare (C++20) | returns true if one range is lexicographically less than another (algorithm function object)
ranges::search (C++20) | searches for the first occurrence of a range of elements (algorithm function object)
mismatch | finds the first position where two ranges differ (function template)

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